This invention broadly relates to a method and an apparatus for manufacturing a glass gob having a predetermined weight by the use of a molten glass. Further, this invention is generally directed to a method for manufacturing a molded glass product by reheating and press-molding the glass gob. Moreover, this invention is concerned with a method for manufacturing an optical device or element by grinding or abrading and polishing the molded glass product.
As a process for manufacturing a molded glass product such as optical glass device or element, there has been known an RP (Reheating/Pressing) method. Such an RP method, a glass gob having a predetermined weight is heated to a predetermined temperature capable of a press molding treatment, thereby performing a press molding treatment to press mold the reheated glass gob. The press molding materials to be processed in the RP method are usually those obtained by cutting a glass plate into a plurality of glass pieces and then subjecting the glass pieces to a rough polishing treatment (called barrel grinding) so as to treat the surfaces thereof. On the other hand, in the process of cutting a glass plate into a plurality of glass pieces for being subsequently subjected to a pressing treatment, it will be difficult for all the glass pieces to have a uniform weight, thus undesirably making one glass piece to have a different weight from that of another. For this reason, in order to ensure a uniform weight for all the glass pieces, a large amount of glass material will have to be removed by virtue of barrel grinding.
In order to solve the above-mentioned problem called weight irregularity, the applicant of the present invention has suggested in its previous patent application an improved method called down-moving cutting, which requires that an mount (predetermined weight) of molten glass be poured into molding dies (which molten glass will be formed into a press molding material), but not involving a cutting process for cutting a glass plate into a plurality of glass pieces (Japanese Unexamined Patent Application Publication No. 2-34525). In such a down-moving cutting process, at first, an amount of a molten glass is poured into molding dies. Then, the movable portion of each molding die is moved down at a speed higher than the flowing speed of the molten glass. In this way, each molten glass flow is cut into separated portions, so that an amount (predetermined weight) of molten glass is left on each molding die, and a gradual cooling may be performed to produce glass gobs which will later serve as press molding material, thereby obtaining glass gobs having various shapes. Usually, a molding apparatus using the down-moving cutting method is equipped with a plurality of molding dies, so that the molten glass may be supplied to these molding dies successively, thereby continuously producing glass gobs.
However, in the above-discussed conventional down-moving cutting method, since the molten glass being supplied to a molding die is in contact with the molding surface of the molding die, it is necessary to avoid the formation of some defects such as through holes and cracks on the surfaces of molded glass gobs. For this reason, in the case where the down-moving cutting method is employed, it is necessary to supply a floating gas such as a nitrogen gas or an air to the molding surface of the molding die, with such a floating gas passing through a plurality of ejection openings formed on the molding surface. In this manner, the molten glass can be maintained in a floated or a slightly floated state, making it possible to minimize the possibility for the molten glass to get in contact with the molding surface of the molding die. In order to satisfy these requirements, the conventional down-moving cutting method has been so improved that the floating gas being supplied from a gas supply source is branched into several gas flows so as to be supplied to the respective molding dies, and that each gas flow is sufficient in its amount so as to effect a sufficient contact between the molten glass and the molding surface of each molding die. In fact, each gas flow is ejected outwardly through the ejection openings formed on the molding surface of each molding die, with such gas ejection being continued from the arrival of the molten glass at the molding surface until the take-out of the molded glass gob from each molding die. In particular, during molten glass supplying process (which will sometimes be referred to as casting process in the following description), since the molten glass is most likely to get in contact with the molding surface of a molding die, the flow rate of each floating gas is increased and this is supposed to be effective for solving the aforementioned problems.
However, in the case where the flow rate of the floating gas has been set in accordance with the aforementioned requirements, and during the casting process for casting the molten glass, the behavior of the molten glass at the moment the molten glass gets in contact with the molding die will become extremely violent, thus resulting in some internal defects (such as foldouts and strias). In order to solve this trouble, the inventors of the present invention have found that it is necessary to perform a control of the flow rate of the floating gas, and that such a control should satisfy the flowing two requirements in order to stabilize a molding state when using the above-described down-moving cutting method.
(1) When the molding dies are used to mold the molten glass, there should be no irregularity in the flow rates of the floating gas flows being supplied to the respective molding dies, and the flow rate of each floating gas flow should be so sufficient that it is sure to float the molten glass from each molding die.
(2) During the casting process of the molten glass, the flow rate of each floating gas flow should be so set that at the moment an amount of molten glass is received into a molding die, the molten glass should have no unstable behavior.